An ONO layer, for example, in which SiO.sub.2, Si.sub.3 N.sub.4, and SiO.sub.2 are successively layered, has been used for the insulating films (dielectric films) of the capacitors provided in a dynamic RAM memory cell.
However, the dielectric ratio for this ONO layer is effectively small, approximately 5; when applied to a large-capacity memory greater than 256 Mb where real estate is restricted, the capacitor dielectric film thickness is reduced, the surface is expanded, and other complex geometrical demands occur, leading to significant process problems.
In this regard, ferroelectric materials having a perovskite crystalline structure have been attracting attention as capacitor insulating film materials for future dynamic RAMS, given their extremely large dielectric ratios ranging from several hundred to several thousand.
Of these ferroelectric materials, a conventional capacitor using PZT, expressed as Pb(Zr,Ti)O.sub.3, is constituted as shown below.
FIG. 13 shows a schematic view of a capacitor; in the drawing, 21 is an insulating substrate, 3 is a lower interconnecting layer (polysilicon), 13 is a barrier metal layer, 6 is a lower electrode, 7 is a PZT film, and 8 is an aluminum upper electrode.
In this capacitor, PZT film 7 is formed using the CVD method (chemical vapor deposition method), the sputtering method, or the sol-gel method; however, since PZT is an oxide, the formation is carried out in an oxygen atmosphere. That is why platinum (Pt) or palladium (Pd) metals with superior oxide resistance are used for the lower electrode 6.
In the past, however, when the formation of PZT on top of a Pt or Pd electrode was attempted, a problem arose when the basic constituent elements, lead (Pb) and titanium (Ti) in the PZT film obtained, were abnormally lacking. Therefore the residual polarization density and coercive electric field strength, which are the performance indices of a ferroelectric material, were extraordinarily degraded.
Upon consideration of the reasons why Pb and Ti were lacking in the PZT film as described above, the following factors were discovered, in accordance with the invention, as a result of material analysis.
First, with regard to Pb, it was assumed that because of the relatively high vapor pressure of Pb at the PZT formation temperature (sintering temperature), the Pb was evaporating off the film surface. However, as a result of analysis, it was found that the Pb in the PZT disperses heavily into the lower electrode and barrier layers, forming electron compounds with these, and uniform dispersion as far as the lower interconnecting polysilicon.
The reason for the lack of Ti, on the other hand, is that, at the PZT formation temperature, the barrier metal Ti interdiffuses with the lower electrode Pd and Pt, forming electron compounds with them and, in the process, the Ti molecule which is missing from the thermodynamically stable phase of the electron compound is supplied from the PZT.
FIGS. 14-20 graphically illustrate these analysis results in detail. It was found, as a result of investigating the phenomena which occur during the PZT sintering process using the SIMS method, that a complex reaction occurs between the lower electrode (Pt/Ti) and the PZT base material during the sintering process, and that a compositional disarrangement occurs between the PZT constituent elements Pb and Ti.
In the analysis, the following measurements (1)-(3) were made.
(1) A compositional analysis was performed using the X-ray diffraction method, heating Pt (film thickness of 200 nm)/Ti (film thickness of 50 nm) and SiO.sub.2 (film thickness of 100 nm) on a Si substrate to 400.degree. C., 600.degree. C., and 800.degree. C. for 1 h in a nitrogen atmosphere. PA1 (2) Using a PZT amorphous powder as a sample formed by heat decomposition at 420.degree. C. of a sol-gel solution made up of Pb (0.1 mol/L):Ti (0.05 mol/L):Zr (0.05 mol/L) and DEA (diethanolamine) (1.4 mol/L), a thermogravimetric analysis (TGA) was performed over the temperature range of room temperature to 900.degree. C., and an investigation of the amount vaporized from the PZT powder was made. PA1 (3) The above solution coated on the SiO.sub.2 /Ti/Pt film was formed on the Si substrate using the spin coating method, then sintered for 1 h at 600.degree. C. to form 2400 .ANG. thick PZT thin film. The concentration distribution of this sample in the depth direction was investigated using the SIMS method. Then, as shown in FIG. 18, the concentration distribution of lead in the depth direction in a dry gel film, an amorphous film, and a 1000 .ANG. thick film of PZT was investigated using XPS. PA1 a) Strength distribution of the Pb secondary ion: an extremely low value of 5-6 arm % is shown at the center of the film depth with respect to the 20 atm % in the Pb prepared composition. Looking at the substrate side, Pb goes through the Pt/Ti layer all the way to the lower layer of SiO.sub.2 and the Si substrate. PA1 b) Strength distribution of the Zr secondary ion: the distribution of the Zr secondary ion shows no dependence on depth; it is uniform. There is also little diffusion in the Pt layer, and considering the nonuniformity of the interface which occurs during sputtering at the time of analysis, the amount of dispersion of Zr in the Pt layer may be ignored. PA1 c) Strength distribution of the Ti secondary ion: looking at the strength of the Ti and Pt secondary ions, it is clear
The respective results of these measurements are explained hereinafter.
Regarding the heat reaction in the Pt/Ti film:
FIG. 14 depicts the temperature dependency of the X-ray diffraction pattern for a Pt/Ti film that is heat treated in a nitrogen atmosphere. As the temperature of the heat treatment rises, the peak strength of the TiO.sub.2 (rutile) (101) diffraction and Pt (200) diffraction increases. In the sample that was heat treated at 400.degree. C., a Ti.sub.6 O.sub.9 (triclinic) (200) diffraction was observed, and in the sample that was heat treated at 800.degree. C., a TiO.sub.2 (rutile) (110) diffraction was observed.
FIG. 15 depicts the X-ray diffraction pattern between 38.degree. and 42.degree.. In the sample that was not heat treated, 39.68.degree. is observed as the peak for the Pt (111) diffraction. In the sample that was heat treated at 400.degree. C., this peak shifts to the steep angle 39.86.degree. side. In the sample that was heat treated at 600.degree. C., it shifts to 39.96.degree., which is assumed to be the (004) diffraction of Pt.sub.3 BTi formed in the Pt--Ti intermetallic reaction. In the sample that was heat treated at 800.degree. C., the diffraction angle was the same 39.96.degree..
As described above, the reaction process which occurs when the Pt/Ti film is heat treated shows a complex temperature dependent behavior.
Regarding the thermogravimetric analysis (TGA) of PZT powder:
FIG. 16 shows the results of a thermogravimetric analysis of PZT amorphous powder formed by heat decomposition of the solution at 420.degree. C. The loss of weight which occurs over the temperature range from room temperature to 200.degree. C. is thought to be the result of separation of the water and CO.sub.2 adsorbed by particle surfaces. No extraordinary change in weight is observed up to 700.degree. C., but above 700.degree. C. a small weight loss is observed. At 900.degree. C. a 0.4 wt % weight loss is observed. Considering that Pb is 63.9 wt % of the entire PbZr.sub.0.5 Ti.sub.0.6 O.sub.2 [subscript legibility is poor], it is clear that even if this weight loss is caused by Pb evaporation, it has almost no effect on the change in Pb concentration in the PZT thin film.
Regarding the SIMS PZT thin film compositional analysis: FIG. 17 shows the secondary ion strength distribution in the depth direction for Pb, Zr, Ti, and Pt as analyzed using SIMS.
that Ti is invading the Pt layer. Furthermore, as the X-ray diffraction results make clear, an electron compound is formed between these elements.
Regarding XPS analysis of the PZT film:
FIG. 18 shows the XPS analysis of Pb concentration in the surface region of the dry gel film, amorphous film, and PZT film formed under different conditions on the Pt/Ti film. Whereas the Pb concentration at the outermost surface is 15-21 atm %, the concentration after etching 5-120 nm falls off precipitously to 5-6 atm %, regardless of the conditions under which the film was formed. It was clear from this that the extraordinary insufficiency in Pb concentration had already occurred by the time the dry gel formation process occurred, that is at a low temperature of 170.degree. C.
The above results led to the following determination, in accordance with the present invention.
Regarding the Pt/Ti electrode reaction:
According to the Pt/Ti equilibriumdiagram shown in FIG. 19, Pt and Ti form electron compounds such as Pt.sub.8 Ti, Pt.sub.3 Ti, Pt.sub.5 Ti.sub.8, PtTi, PtTi.sub.3.
Regarding the reaction between PZT and the Pt/Ti film:
The behavior of Pb was considered based on the Pt-Pb equilibrium diagram shown in FIG. 20. Below 290.degree. C., Pt and Pb form electron compounds such as PtPb.sub.4, PtPb, and Pt.sub.3 Pb. According to the analysis results from the XPS mentioned above, Pb diffuses toward Pt at 170.degree. C., the temperature at which the dry gel is formed, but according to the SIMS results, it is not accumulated in the Pt
This suggests that at 600.degree. C., PtTi.sub.x is more layer. thermodynamically stable than PtPb.sub.x.
Regarding the phenomenon by which the Pb concentration is lower than the prepared composition, it is generally thought that, during sintering, this occurs as evaporation from the PZT film surface in the form of Pb or PbO. However, the amount evaporated from the PZT surface is minute, and judging from the results above, almost all of it is diffused into the substrate. Furthermore, from the fact that Pb is detected in the Si as well, the Pb which passes through the Pt/Ti layer goes into solid solution in the SiO.sub.2, and further diffuses in the Si substrate.
Ti behavior can be surmised from the Pt--Ti equilibrium diagram shown in FIG. 19. The Pt and Ti in the substrate primarily form Pt.sub.8 Ti under heat treatment. When a thin film of PZT is sintered on top of the Pt/Ti film, it appears that an electron compound of Pt and Ti is formed, while at the same time the Ti is absorbed in the Pt layer in the PZT film, forming an electron compound. Therefore in order to prevent a diminution of Ti concentration in the PZT film, it is necessary to make the Pt film thickness as absolutely thin as possible, and to consider the scaling of Ti when establishing the prepared composition of sol-gel base material.
Above, the SIMS, XPS, and XRD analysis of the reactions which occur during the process of formation of PZT thin film on a Pt/Ti film have been discussed. To summarize, the composition of PZT film formed by the sol-gel method, in particular, is greatly influenced by the diffusion of Pb into the substrate side and the reaction between Ti and Pt which accompanies heat treatment. In particular, the Pb diffusion phenomenon is extraordinary at a temperature around 170.degree. C. The decrease in Ti concentration is caused by the alloy reaction which occurs with Pt during sintering. Also, the Zr in the PZT film is extremely stable with respect to the Pt layer.
It is an object of this invention to provide a capacitor, an electrode structure, and a semiconductor memory device capable of suppressing fluctuations in the composition of the ferroelectric layer in PZT, etc., so as to maintain its intended performance, of simplifying and stabilizing film fabrication, and of preventing the degradation of electrical characteristics and adverse effects on lower layers.